Focusing Screens

Focusing screens can be made of a number of flat translucent materials but most commonly are formed from ground glass and are found in a system camera to preview a framed image in a viewfinder. The etchings in the focus screens tends to be different from one model to another, with designations as to what to expect from each one.

Focus screens have been around almost as long as cameras, with some of the first primitive cameras being nothing more than a box with a board to hold the lens in front and the focusing screen in the back, which was replaced with some sort of imaging medium like a plate prior to snapping the picture.

A clean focus screen is used to help determine if the targeted shot is actually in focus, with mechanisms like a microprism ring blurring the focus until the lens is set correctly or a split screen that only allows for the fully focused image to be seen when the configuration of the camera lens is focused enough to bring the two halves of the screen together.

In professional cameras today the photographer often has a choice of screens that can be fairly easy to replace in most models. These DIY focus screens are fairly specific to applications. For example, if the photographer is shooting with a wide angle lens for landscaping or architectural prints, he or she will opt for a focusing screen that has a grid etched on it to help control perspective distortion, while low light situations call for a plain screen. If there is a need to focus quickly, a split screen will be chosen. These are just a few of the options available.

One of the major brands manufacturing camera with interchangeable screens is Canon. A Canon focus screen can come in several different configurations for various uses as described above. Some of these cameras have interchangeable film formats (like medium format cameras), especially the high end models, and these often have etchings on the screen to show the film limits. Canon focus screens used in these cameras are typically plain or have a grid etching because, with the size of the focusing screen meaning that the only aid required to really focus being a magnifying glass.

Because focusing screens have become an integral part of most cameras, only professional photographers usually find themselves changing from one type to another. In other cameras, the basic focusing screen required is already incorporated into the camera.